The present invention relates to a loop transmission control system, and more particularly to method and apparatus for controlling transmission of a token frame signal in a loop transmission system by a token passing system.
A token passing system has been known as a data transmission system which prevents frames sent from data transmission control units from conflicting each other on a transmission path in a loop transmission system having a plurality of data transmission control unit or one node unit connected through one or more loop transmission lines. In the token passing system, a frame including token indication indicative of a transmission right is circulated through the loop transmission line, a transmission control unit having no transmission request relays the frame, and a transmission control unit having a transmission request waits for the token frame which will be circulated and, when it arrives, sends a message frame while it holds the token frame. The token frame held by the transmission control unit is again sent to the loop transmission line after the message frame has been sent. The transmission control unit which sends the token frame following to the message changes a source address of the token frame to an address assigned to its own unit.
In the loop transmission system which employs the token passing system, a leading portion of the token circulated through the loop conflicts to a main portion or trailing portion of the token being sent depending on a relationship between a length of the loop transmission line and a data delay time through the data transmission units so that the relay of the token in the sending unit is incomplete and the token may be lost on the loop. In the loop transmission system, if there is no data transmission request from any terminal unit connected to the transmission control unit, the token frame is circulated through the loop transmission line in a minimum delay time. Accordingly, in a loop transmission system having a relatively short loop transmission line length, if the transmission control unit which has just sent the message frame attempts to release the token, the leading portion of the token circulates through the loop before the token frame has been sent and it is not transmitted because it overlaps with the transmission of the main portion or trailing portion of the token. When the sending unit relays the received frame after the transmission of the token frame, the token frame is partially lost and the token frame is eventually lost on the loop. In JP-A-57-81746, in order to prevent the loss of the token frame, when a transmission control unit which is sending a token frame receives the token frame which it has sent, the received token is aborted in anticipation of the loss of the token even if a complete form of token frame has been actually relayed, and the token frame is again sent. However, in the prior art system, since there is a large delay time before the complete form of token frame circulates through the loop transmission line, the other transmission control unit on the network takes a long time to get the token if a data send request occurs in the other transmission control unit.